1. Field of the Invention
The present invention relates to an organic light emitting display and a driving method thereof, and more particularly, it relates to an organic light emitting diode (hereinafter, ‘OLED’) display and a driving method thereof.
2. Description of the Related Art
In general, an OLED display electrically excites phosphorus organic components, and visualizes an image by voltage-programming or current-programming M, X, and N numbers of organic light emitting cells. These organic light emitting cells include anode indium tin oxide (ITO), organic thin film, and cathode (metal) layers. The organic thin film layer has a multi-layered structure including an emission layer, an electro transport layer (ETL), and a hole transport layer (HTL) so as to balance electrons and holes and thereby enhancing efficiency of light emission. Further, the organic thin film separately includes an electron injection layer (EIL) and a hole injection layer (HIL).
A method of driving the organic light emitting cells having the foregoing configuration includes a passive matrix method and an active matrix method, the active matrix method employing a thin film transistor (TFT). In the passive matrix method, an anode and a cathode are formed crossing each other, and a line is selected to drive the organic light emitting cells. However, in the active matrix method, each indium tin oxide (ITO) pixel electrode is coupled to the TFT and the light emitting cell is driven in accordance with a voltage maintained by the capacitance of a capacitor coupled to a gate of the TFT. Herein, the active matrix method is classified as a voltage programming method or a current programming method depending on the type of signals transmitted to the capacitor so as to distinctively control the voltage applied to the capacitor.
FIG. 1 is an equivalent circuit diagram of a pixel circuit according to a conventional voltage-programming method. A conventional OLED display device employing the voltage-programming method supplies current to an is OLED display through a transistor M1A coupled thereto for light emission. The amount of current supplied to the OLED is adjusted by a data voltage applied through a switching transistor M2A. Herein, a capacitor C1A is coupled between a source and a gate of the transistor M1A to maintain the amount of the data voltage applied for a predetermined time period.
When the transistor M2A is turned on, the data voltage is applied to the gate of the transistor M1A, and a voltage of VGS between the gate and the source is charged to the capacitor C1A. A current IOLED flows corresponding to the voltage of VGS, and the OLED emits light corresponding to the current IOLED.
Herein, the current flowing to the OLED is given as Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where IOLED represents a current flowing to the OLED, VGS represents a voltage between the gate and source of the transistor M1A, VTH represents a threshold voltage of the transistor M1A, VDATA represents a data voltage, and β represents a constant number.
As shown in Equation 1, the current corresponding to the data voltage is supplied to the OLED, and the OLED emits light corresponding to the current supplied thereto. Herein, the data voltage has multi-leveled values within a predetermined range to express gray scales.
However, a pixel circuit according to a conventional voltage-programming method has a problem of expressing high-level gray scales due to deviation of the threshold voltage VTH of the TFT and mobility of a carrier, the deviation being generated as a result of a non-uniform manufacturing process of a TFT. For instance, when the pixel circuit drives a TFT of a pixel using 3V to express 8-bit gray scales (256 gray scales), a gate of the TFT must be applied with a voltage at an interval of less than 12 mV (=3V/256). However, it is difficult to express a high gray scale in the case that the deviation of the threshold voltage VTH is 100 mV due to the non-uniform manufacturing process. Moreover, the deviation of mobility causes the value of β to be changed in Equation 1, and thus expressing the high level gray scale becomes more difficult.
On the other hand, although the amount of current and voltage supplied from a driving transistor in each of pixels is not uniform, the circuit of pixels employing the current-programming method can provide panel uniformity as long as current supplied from a current source to the pixel circuit is uniform.
FIG. 2 shows an equivalent circuit diagram of a pixel circuit according to a conventional current-programming method. A transistor M1B is coupled to an OLED to supply a current for light emission, and the amount of the current is adjusted by a data current applied through a transistor M2B.
Accordingly, when transistors M2B and M3B are turned on, a voltage corresponding to the data current IDATA is stored in a capacitor C1B. The amount of current corresponding to the stored voltage flows to the OLED so that the OLED emits light. Herein, the current flowing to the OLED is given as Equation 2.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                                                                V                      TH                                                                                          )                            2                                =                      I            DATA                                              [                  Equation          ⁢                                          ⁢          2                ]            
where VGS represents a voltage between a gate and a source of a transistor M1B, VTH represents a threshold voltage of the transistor M1B, and β represents a constant number.
As shown in Equation 2, the current flowing throughout a panel can be uniform since the amount of the current IOLED flowing to the OLED is the same as the amount of the data current IDATA according to the conventional current-programming method. However, a little current (IDATA) flows to the OLED, and thus it takes too much time to charge data lines. For instance, assume that the load of capacitor in the data line is set to be 30 pF. In this case, it takes several milliseconds to charge the load of the capacitance with data currents of several tens of nA to several hundreds of nA. However, line time is inefficient for full charging of the data line since it is limited to several μs.
Moreover, increasing the amount of the current IOLED flowing to the OLED so as to reduce time consumed for charging the data line may cause the brightness of all pixels to increase, thereby resulting in a decrease of image quality.